There's no question that walking is good for you. It's free. It's easy to do. And it's easy on your muscles, joints, and bones. Walking is an aerobic exercise. It improves your body's utilization of oxygen. It lowers the risk of blood clots and heart attacks. Walking is one of the easiest ways to increase your physical activity and improve your health.
But there's more. Walking can also improve your posture. As someone who's had less than perfect posture all his life, I'm discovering that my daily walks have helped me to improve my posture by leaps and bounds. The simple act of walking breaks my slouching posture for a while. And no, you won't end up with the infamous "stand straight, shoulders back" posture that, if you look around, nobody actually has. You'll end up with a normal posture, which is a little curved but not slouching, which is normal because your spine is a little curved.
One final tip I have is this. It's almost impossible to walk with your shoulders slouching forward if you hold your hands with their palms facing forward. I do this for a few minutes every time I walk. Try it and see what I'm talking about.
Good athletes have good posture because it works.